1. Field of the Invention
This invention relates to a control system for a hybrid vehicle, and in particular, to a control system for a hybrid vehicle which reduces a shock when the vehicle shifts from a cruise mode for generating electric power without assisting the driving shaft of the engine to an acceleration mode for assisting the engine.
This application is based on Japanese Patent Application No. Hei 10-359831, the contents of which are incorporated herein by reference.
2. Background Art
Conventionally, hybrid vehicles which carry motors as power sources for driving the vehicles in addition to engines are known. Hybrid vehicles are divided into series hybrid vehicles and parallel hybrid vehicles. In series hybrid vehicles, the engine drives a generator, which outputs electric power to the motor, and the motor drives the wheels.
Because the engine and the wheels are not mechanically connected, the engine constantly runs within a rotational range which reduces fuel consumption and emissions, as compared with conventional engine vehicles.
In parallel hybrid vehicles, the motor connected to the engine assists the rotation of the drive shaft of the engine while charging a battery using a generator which is separate from the motor or is the motor itself.
Although the engine and the wheels are mechanically connected, the parallel hybrid vehicle can reduce the load to the engine. Thus, the parallel hybrid vehicle also has reduced fuel consumption and emissions as compared with conventional engine vehicles.
In the parallel hybrid vehicle, the motor for assisting the output of the engine is directly connected to the output shaft of the engine, and acts as a generator for charging the battery when the vehicle speed is reduced. Alternatively, either or both of the engine and the motor may generate the drive power, and the generator may be provided separately.
When a hybrid vehicle accelerates, the engine is assisted by the motor, and, when it decelerates, various operations are performed, such as charging the battery by deceleration regeneration, so as to maintain sufficient electric energy in the battery (hereinafter referred to as the "state of charge (remaining charge)") to meet the driver's demands (as disclosed in, for example, Japanese Patent Application, First Publication No. Hei 7-123509).
In conventional hybrid vehicles, particularly, in parallel hybrid vehicles, when the vehicle is cruising, the motor does not perform the torque assist for the driving shaft of the engine, while the motor acts as a generator for generating electric energy to increase the remaining charge.
However, when the generation is started during the cruising, the driver may depress an accelerator pedal so that the vehicle enters the acceleration mode, which may cause a shock.
When the vehicle enters the acceleration mode, the motor normally performs the torque assist, that is, adds a torque to the torque from the engine. When the motor as the generator generates, i.e., 2 kW during the cruising and performs the assist of 2 kW in the acceleration mode, a variation of 4 kW in total occurs in the output. This variation in the output causes a shock greater than anticipated when the vehicle accelerates, and the driver may feel uneasy.